text:
In Raymer diagram "Take off parameter- Take off distance", what is the difference among "balanced field length", "over 50 ft", "ground roll"?
Raymer in his book suggests a method to find wing loading W/S (for example we consider jet with 2 engines) (starting from specific thrust $\frac TW$, $CL_{to}$, density/reference density etc.).
In step 1:  he suggests to use graphics "Fig. 5.4 Takeoff distance estimation": you enter takeoff distance (in ordinate), so .. in abscissa you can determine Takeoff Parameter ("TOP")
Step 2: From "TOP" you can find $\frac WS=TOP\cdot \sigma \cdot CL_{to} \cdot \frac tW$
The questions are: in figure 5.4 you can find many diagrams: takeoff distance for:
a) "Balanced field Length"
b) "over 50 ft"
c) "ground roll"
Which one I have to use?
What is the difference among them?
Which curve you have to use, in order to calculate "take off parameter"?
Source: Daniel P. Raymer, Aircraft design: a conceptual approach. Chapter 5 Thrust-to-weight ratio and wing loading, paragraph 5.3 Wing Loading, sub-paragraph Takeoff Distance, page 88-89 and "Fig. 5.4 Takeoff distance estimation".

I noticed that you often ask about Raymer, so I assume youtook it as a sort of "reference book" for Aircraft Design.
I would suggest you to look at other books for completeness, for example:  Roskam "Airplane Design" and Torenbeek "Synthesis of Subsonic Airplane Design", Nicolai "Fundamentals of Aircraft Design".

Let's clarify:
Large commercial airplanes take off in a way which ensures there is sufficient runway length to stop in case of aborted takeoff. There is a speed threshold, V1 speed or decision speed, that determines whether the takeoff can still be aborted, or the airplane must continue and liftoff whatever happens. See balanced field takeoff on Wikipedia.
The balanced field length is the minimum runway length to execute a balanced field takeoff for a given aircraft. The V1 speed in this case is the lowest possible. Additional runway length allows greater V1 to be selected (greater V1 means a greater portion of the ground roll with a possibility to abort the takeoff).

V1 value and balanced field length (source)
50 ft distance: This is the distance from the start of the ground roll to the point where the aircraft is airborne and 50 ft over the runway, to ensure obstacle clearance. 35 ft is another typical obstacle clearance in the US, and indeed the distance to climb to 35 ft is shorter than the previous one (a 35 ft clearance is implicitly used to compute the balanced field distance)

Takeoff roll distance and Obstacle clearance distance (source)
Using the diagram
From this slideshow which reproduces the figure you refer to:

(Source)
This depicts the relationship between takeoff parameter and different distances, either the ground roll distance or the obstacle clearance distance (35 and 50 ft) in the three types of takeoff described above.
There are three groups from left to right:

Jet using balanced field T/O (the obstacle clearance is implicitly 35 ft).
Jet not using balanced field T/O.
Propeller (not using balanced field T/O).

In these groups, there are one or two data provided:

Jet using balanced field T/O. The only distance provided is the balanced field length (distance to accelerate, abort the T/O at V1 and decelerate). You must choose the right curve, depending on the number of engines (twin, tri or four-engine).
Jet / propeller. There are two distance provided and you may need both. Ground roll is the distance until the aircraft leaves the ground (liftoff); 50 ft is the ground roll + the distance to climb to 50 ft.

See also:

What would be the ground roll and total distance to clear a 50ft obstacle given these conditions?
How is minimum runway length related to V1?

Thank you Mins for your wide explanation. I have to add a question in order to understand better: if jet is not using balanced field t/o how can be found the values (couples of takeoff distance and takeoff parameter) depicted for "Ground Roll" and "Over 50 ft" you can find in proper curve? Which different aircraft conditions you have in stead of balanced field t/o conditions (or hypothesis) ?

@d.pensopositivo: Not sure I understand fully the comment. If this is a jet not using a BF T/O, then you look at the group in the middle (5th and 4th curves from the left). You know your T/O parameter, report it on the horizontal axis, find the projection onto the two curves, report the intersections onto the vertical axis. This gives you the ground roll distance (5th curve) and the 50 ft obstacle clearance distance (4th curve). Feel free to ask more clarification if required.

Thank you; to understand better, do you know in which case the jet aircraft is not using balance field takeoff? In such case, historically which data the curve "ground roll" are taken from? How are they originated?

@d.pensopositivo: Balanced field T/O is required for the large commercial transport aircraft category in the US (CFR Title 14 → Chapter I → Subchapter C → Part 25 → Subpart B → §25.107 et seq.) Distances are taken from the certification test flights and are included in the AFM.


text:
Do transoceanic airliners monitor marine VHF emergency frequencies?
I know that modern jetliners often have a multitude of radio equipment on board, including VHF, HF, satellite, etc.
From what I've read (such as here), it's customary for jetliners to monitor 121.5 MHz for aircraft emergency traffic.
Do commercial airliners traveling transoceanic routes also monitor marine VHF channel 16 (156.8 MHz), a common frequency used by ships in distress?
If so, is this a fairly customary, routine matter (like monitoring 121.5 MHz) or is it simply up to the crew of each aircraft?

"From what I've read, it's customary ".  Just wondering where you read that if you remember.

@user6035379 Good question. I don't recall, exactly. I have a tendency to browse Wikipedia and Stack Exchange with many tabs, but it was most likely on one of those two sites.

156.8 MHz
Crews don't listen to channel 16 of the marine band.
Channel 16 is used by marine with FM, while a VHF aviation receiver demodulates only AM, and the upper limit of the VHF band for aviation is 137 MHz.
Channel 16 is monitored by local marine agencies -- coast guards -- e.g. CROSS in France and USCG in the US.
Aircraft are only required to carry an equipment able to receive and transmit on emergency frequencies used by aviation.
Emergency frequencies other than channel 16 are also used in marine, including 121.5 MHz, e.g. by EPIRBs. The use of 121.5 MHz for marine EPIRBs was to allow monitoring by aircraft.
Air regulation for listening 121.5 MHz
EPIRBs monitoring by aircraft is not relevant any more as EPIRBs are now efficiently monitored by satellite (COSPAS-SARSAT). However 121.5 MHz is still monitored ("guarded") by aircraft, in particular for interception response purposes. The requirement is specified in ICAO Annex 10 (volume II) to the Chicago Convention:

5.2.2.1.1.1 Aircraft on long over-water flights, or on flights over designated areas over which the carriage of an emergency locator
  transmitter (ELT) is required, shall

continuously guard the VHF emergency frequency 121.5 MHz , except for those periods when aircraft are carrying out communications on
  other VHF channels or when airborne equipment limitations or cockpit
  duties do not permit simultaneous guarding of two channels.

5.2.2.1.1.2 Aircraft shall continuously guard the VHF emergency frequency 121.5 MHz in areas or over routes where the possibility of
  interception of aircraft or other hazardous situations exist, and a
  requirement has been established by the appropriate authority.
5.2.2.1.3 Aeronautical stations shall maintain a continuous listening watch on VHF emergency channel 121.5 MHz during the hours of
  service of the units at which it is installed.

ICAO standard and recommended practices ("SARPs") for international aviation activity are recommendations, but they are generally implemented in the national regulations by the member states.
406 MHz
121.5 MHz is being replaced by 406 MHz, as this is the frequency of the COSPAS-SARSAT organization. This organization provides a permanent monitoring worldwide through the use of polar orbit satellites augmented by geostationary satellite relays.

Couldn't say it better

Fantastic answer that answers my question precisely.

On top of all this, a boat in trouble in the middle of the ocean would not likely try vhf unless there was a ship in sight, since it has limited range. There are a number of marine bands in the hf (shortwave) range which contain designated distress frequencies monitored by coastal stations, on which there’s a chance of being heard over long distances. But today, of course, one of the satellite communication methods would probably be best.

This answer is slightly offtopic for the aviation bit, but I believe it is useful to answer the question.
Marine VHF at 150-ish MHz is a local radio technology. It reaches the horizon, no further.
With an aircraft, the horizon can be hundred of miles away, due to the height of the aircraft. At 10000 m height, the horizon is approx. 350 km away. At 40 m (a reasonable height for antenna on a vessel), it's 25 km away. So while an aircraft at cruising altitude can use Line Of Sight-frequencies to communicate with another aircraft 600 km away, a vessel is restricted to 30-50 km distance on the very same frequencies.
A vessel in the midst of the ocean would not use VHF to send a distress signal. It may use VHF to communicate with rescue vessels within sight, to coordinate the rescue. To send a distress signal, it would use

Satellite communication systems
High Frequency Radio.

HF radio uses approx. 1.6M Hz to 30 MHz. The distress frequency is 2182 kHz. All ships above 300GT are required to listen to 2182 kHz:

In order to provide an effective link between the international radiotelegraph distress frequency (500 kHz) and the radiotelephone distress frequency (2,l82 kHz) all ships subject to the requirements for radiocommunications under the SOLAS Convention (basically this means cargo ships of 300 gross tonnage and above and all passenger ships) should be fitted with radiotelephone equipment and be required to keep a continuous listening watch on 2,182 kHz.

Whilst 2182 kHz probably does not have global reach, it can propagate as a ground wave well beyond the horizon. Other frequencies on the HF band, 20-22 MHz, may very well have a truly global reach, especially at night.

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text:
Could you make a quadcopter whose propellers can also work as wheels?
I was thinking of designing a quadcopter drone that could both fly and move on land thanks to its propellers being placed incidental a ring like the ring props used in boats. Would this be possible and/or effective?

like this?

@linkin has shown that it's possible. Effective? That depends on what you mean by 'effective'. Do you want to create a Roomba-like cat transporter that flies? Or drop bombs on soldiers in Ukraine?

Beware that there is a chasm between 'designing', which is what is asked in the question and the rest of a product lifecycle; that includes buildability, reliability, maintainability and other factors. Key considerations are the vastly different rotation speeds and aerodynamics versus physical strength requirements for the rotors/wheels. You might not meet the bar for 'effective', depending on whether and how this design would be built and used.

Note: the outside wheel should not turn on air, else you lose much power. On car you may see it on wheel size, but they move slower. (Angular momentum). For sure you may find good light materials, but so much more costly (and you lose power). The answer is about SAR so possibly optimized for size (an aircraft: you optimize for weight, volume is not so important).

A very comprehensive study into what you are describing is published in Nature Communications under the exciting title "Multi-Modal Mobility Morphobot (M4) with appendage repurposing for locomotion plasticity enhancement" - the same drone as in the video @Linkin linked to.

source
It is envisaged that such a device could be used in Search and Rescue, although at only 5kg the rescue part might be a bit optimistic.

Nature articles are peer-reviewed, the reviews are also interesting.

Looks good in principle, but I see a lot of places where devils can hide in the actual engineering details. Mostly I'm thinking of the conflict between the need for components to be sufficiently light in weight for the thing to fly, but also, sufficiently robust for travel over the ground.  It also may be tricky to design a suspension that can keep the wheels in contact with the ground if the whole vehicle is light enough to fly.

@SolomonSlow If they are actually designing it in such a way that it could ‘walk’, the suspension issue could be at least partly solved with some good sensors and real-time active reconfiguration.

"Rescue" includes keeping trapped individuals alive while heavier equipment clears a path to them. Small robots can help in this role by providing hope (letting the trapped person know that there are people looking for them, and providing a communications channel) and transporting small packages of nutrients/water/medication.

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text:
Does the "Maintenance Indicator" sign on a METAR imply we can't trust it?
As explained in this Q/A:
What does a dollar sign mean in a METAR?
The presence of a Dollar Sign ($) at the end of a METAR is

included when an ASOS/AWSS detects that maintenance is needed on the
  system.

Does this therefore imply we should not trust this METAR? If so - why do they even continue to publish it? If not, why are pilots informed that the system requires maintenence?

Not necessarily.
Automated stations have a number of sensors for detecting,

Wind speed and direction
Visibility
Obscurations to vision
Cloud coverage and ceiling
Temperature and dew point
Barometric pressure and altimeter setting
Precipitation
Freezing rain (Icing)
Lightning (thunderstorms)

It is likely that some of these are not functioning properly, while the others are working correctly. In such cases, the METAR reports contain sensor status indicators which give information about the sensor which has failed. For example, consider the following METAR:

KMEM 230853Z AUTO 18014G18KT 10SM CLR 16/M02 A3008 RMK AO2 SLP117
       T01561022 TSNO $

Here, TSNO indicates that the automated station is equipped with a lightning detection system and that sensor is not operating. Following is a list of sensor status indicators:

RVRNO- Runway Visual Range is missing.
PWINO- Present weather identifier sensor (is fitted and) is not operating
PNO- Tipping bucket rain gauge sensor is not operating.
FZRANO- Freezing rain sensor is not operating
TSNO- Lightning detection system sensor is not operating
VISNO_LOC- Secondary visibility sensor is not operating.
CHINO_LOC- Secondary ceiling height indicator and that sensor is not operating.

So the dollar sign would follow  the specific item for which the sensor is not working, and it's not simply tacked on the end to indicate "something's broken"?

Well, the dollar sign is always the very last item in the metar, if something needs maintenance (not necessarily "broken").  The "thing that is broken" is not always indicated (see the link in the question - nothing is indicated). Those 7 items in the answer come from AC 00-45H, Para 3.1.5.14. If it is one of those things that is broken, then that will be the very last thing in the remarks section, immediately followed by the dollar sign.  So yes: the dollar sign will follow the specific item that is broken (if it is listed), AND the dollar sign is always "tacked on to the end".

Additionally,  some elements in the report may simply be omitted when the sensor is down, without reference in remarks. This happens most often with wind components and temperature/dewpoint items.

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text:
Split flap variant
Reading about this Q&As, how would a split flap behave if it acted like a solid, with no real split, but a thick trailing edge?
Mostly wondering what happens between a) and c) and influence of that reflex airfoil looking shape.
Not sure if position b) increases lift at all?
At position d) is it equivalent to a split flap?
Between d) and e) it looks like a zap flap, chord increase yet camber decrease, so deploying further may not increase lift anymore?
Wondering if position e) has flow reattaching to flap?

Intriguing idea, and thanks for the animation!  I'm looking forward to a good answer from the aero pros, but my hunch is that it ought to perform somewhere in the middle of split and fowler flaps at full deflection.  However, there would be a big drag penalty at half flaps, so intermediate settings wouldn't help take-off performance.

Can the animation be paused?  Or replaced with a few still images from zero to full deflection?  I find it hard to type even a comment nevermind a full answer while my eyes get constantly yanked back to the picture.

Wow... now the animation is so slow I sat there looking at it wondering what animation y'all were talking about...

All of your flaps, starting with position b, will add lift, the more the further back they are positioned. Again, I will use Sighard Hoerners book Fluid Dynamic Lift to support my point. In Figure 9 of chapter 5 he plots the added lift as a function of flap position:

Your most rearward flap is a Zap flap, except for adding a solid panel between flap and trailing edge. I'm afraid that adding the panel will achieve nothing, except for adding some mass. Since the gap between flap and wing is already filled with separated flow and the space aft of that added panel will also contain separated flow, the aerodynamic effect will be zero. The flow around the wing will look exactly the same, with or without this panel.


text:
Additional type of entering to holding pattern
Does anyone know any other types of entering to holding pattern besides the standard ones?
I’m trying to become a cpt and our local comission asked this question to me. They said that there is an addition 4’th type. But I know only three.

This depends on local fight rules.  ICAO allows a "Secondary Fix" entry, which is similar to an offset/teardrop entry, except the instruction is to fly to the secondary fix first, rather than the holding fix first.
FAA allows for "other entry procedures" that are not among the three recommended entries.  Keep in mind, ATC does not always issue holding instructions in combination with a direct course to a holding fix.  An aircraft on a non-radial heading away from a fix in space could be instructed to "turn right and hold" at such fix.  Depending on the present position, heading, speed, turn radius, published hold or airway, and navigation equipment, the pilot would have to design an entry appropriate for that scenario.
References:
Conventional Holding (ICAO)
AIM 5-3-8 (FAA)


text:
How does lift depend on velocity in 3D?
Many resources provide the following type of cross-section view when explaining aerodynamic forces:

The textbook says that $V_a$ is the speed of the aircraft through the surrounding air mass.
Does this speed include the velocity component perpendicular to the surface of this cross-section view? (i.e. velocity component coming out of the paper)
For example, suppose that there is no wind and the velocity of the aircraft is given by
$$\begin{bmatrix}u \\v \\w\end{bmatrix}$$
in body coordinates (front-right-down). If I want to calculate the lift of the main wing, what is $V_a$?

$\sqrt(u^2 + v^2 + w^2)$

or

$\sqrt(u^2 + w^2)$

Reading through the textbook seems to indicate 1. but this open source simulator
projects the velocity vector onto the lift-drag plane (XZ plane) which indicates 2.
Figure was taken from "Small Unmanned Aircraft: Theory and Practice, by Randy Beard and Tim McLain"

This can become a very long debate, since we are between two worlds here : real physics and simulation computations. To get out, you should define first what kind of optimization you want for device/computer performance reasons. That's why the 2) is the way to go for simulators, many other "minor" forces/vectors are ignored aswell. If you want to be as realistic as imaginable, you would compute everything, even swirling gusts and air masses/temperature, but good luck finding the machine that would cope with, especially if you intent to have beautiful sceneries and stunning weather effects...

The code you are using needs the velocity as given in your equation 2. because this is how lift and drag are defined/calculated in that code. The velocity component "coming out of the paper" is not needed because there's the implicit assumption that it is negligible in respect to the other two components. This assumption normally true because any airplane is laterally stable i.e. it always tends to align with the freestream given by $u$ and $w$. Only if the airplane is flying in a sideslip then also $v$ becomes important and other aerodynamic coefficients have to be taken into account as well.

Also, the curvature in that direction is typically 0 or near zero, so any crossflow will generate minimal forces.


text:
Calculate distance from airplane to me using adsb
I use rtl-sdr dongle and dump1090 to get nearby airplanes. I get adsb SBS message. Now my question is that how can I calculate distance from an airplane and my rtl dongle?
In sbs message we have generation time and logged time. Is difference of this two value reception time or not? If is it true, can we calculate distance with this values? If yes, how?

Skimming through https://mode-s.org/decode/ I think the answer is "no." I don't think ADS-B messages include a timestamp. You can determine an aircraft's position using multilateration if you receive the same exact message at geographically distinct receivers; you can mark when each receiver got the message and work backward from there. But I don't think you can do it with just a single receiver.

Of course the ADS-B message also includes the aircraft's position (that's the entire point) and if you know your receiver's position, which you should, you can calculate the difference that way...

Thanks @randomhead, can I calculate my rtl dongle location using multilateration?

Not easily, if at all... but your receiver should be in a fixed location, generally speaking, right? And if it isn't, won't you be there with it and you can use a GPS device to report your current position? Or are you planning on sending your SDR dongle out on its own in a weather balloon or something? Also: there are USB GPS dongles that you can plug into your computer alongside the SDR.

no, I must find distance or my location without any GPS dongle in my project only with a rtl-sdr dongle. do you know any solution that you say is not easy?

@UKRock I think you're not barking up the right tree here. If you don't know where you are, you're going to have to figure that out first. There is no way to trilaterate unless you have ToF between at least two spatially separated receivers. And those receivers have to be time synced to nanosecond resolution. That's way, way, way more complex than having a GPS.

Regarding trying to use a a timestamp, the level of timing resolution AND time syncing you would need to calculate distance based on ToF (Time of Flight) is nanoseconds. That's just not reasonable, esp. since there is no mechanism for providing absolute timing on the ADSB side.
Instead, I would suggest just using google maps to figure out your personal lat-lon, and then use an online calculator to convert that to distance.
Formally, you want to use the haversine formula.
In addition to the online calculator, https://www.movable-type.co.uk/scripts/latlong.html has all the equations you will need for a variety of solutions. It also has sample code, which might make it easy to automate the task.

Thanks for your answer, but I dont have my dongle location to calculate distance. Because my project does not have any user to check google map and calculate in online websites. Maybe if I ask how to calculate my location with sbs messages, is better!

@UKRock it's imperfect, but geolocation via IP address, e.g. https://www.iplocation.net/, can serve in a pinch.


text:
Calculate the radius, speed and sink rate of a glider with a specific bank angle in a steady turn
For a glider, what is the radius, speed and sink rate of a glider with a specific bank angle in a steady turn while flying at minimum sink speed -> speed where $\frac{{c_L}^3}{{c_D}^2}$ is max.
I came across some similar questions, but none of them answered it for me.
What I got is a glider with constant gravitational force $g$, wing area $S$ and mass $m$. I also know the minium sink velocity $u_{sink_{min}}$ for a given wing loading and the corresponding speed $u$. The $c_L$ to $c_D$ graph is also known.
I know that the radius is calculated with:
$$ r = \frac{{u_{turn}}^2}{g \cdot tan(\phi)}$$
What I cannot get my head arround is the fact, that the velocity $u_{turn}$ needed in that turn is also dependend on $\phi$. How is the velocity $u_{turn}$ calculated than?
My first thought was that it is dependend on square root of the load factor $n=\frac{1}{\cos(\phi)}$. Because of the higher wing loading/load factor, the needed lift $F_L$ is higher and so you must fly faster.
And how can I calculate the $u_{sink_{min}}$ (sink rate) in a turn than ? Is it as simple as:
$$u_{sink_{turn}}=u_{sink_{min}} \cdot \sqrt{\frac{1}{\cos(\phi)}}$$
It felt too easy, thats why I ask you guys to help me solve this problem for my thesis.
I know that $F_L=F_G \cdot \cos(\gamma)$, but for simplicity and small glide paths in gliding I assumed that $\cos(\gamma)=1$.
The second question to truly solve this question is to also take the uplift/winds in consideration because updrafts varies with distance from the center of the thermal. Maybe someone also has a smart idea for that.
Many greetings and thanks to all of you in advance
My used values (ASK23):
$m=330$;
$g=9.81$;
$\epsilon=0.93$;
$AR=17.44$;
$c_{D0}=0.0069$;
$S=12.9$;
$u_{sink_{min}}=0.66$ @74km/h in straight flight
Edit 1:
I plotted $v_{sink}$ over $\phi$ with Peter Kämpf first formula. The graph looks rather unrealistic.

Edit 2: I plotted $v_{sink}$ over $\phi$ again with Peter Kämpf editted formula. The graph looks better now, but I still think the sink rate is way to low. In reality much more people would fly with a higher bank angle when the degredation of the $v_{sink}$ is like that in that graph.

After looking at this graph for awhile, i realised that my first mathematical approach with $u_{sink_{turn}}=u_{sink_{min}} \cdot \sqrt{\frac{1}{\cos(\phi)}}$ looks rather similiar.

From my own soaring experience in an ASW20, the bank angle is mostly determined by the diameter and strength of the thermal more than that kind of mathematical analysis.  Once I think I'm near the core, or if the thermal is small, I normally go right to 45 deg or a little more, and I go a bit faster than min sink, around 48-50kt and trim it out. I find it settles into a groove that is easy to maintain and I get better climb performance. If I have to I'll go right to 60 deg to stay in the core lift. In other words, the smallest circle that can be flown consistently.

@RalphJ in general though once you are cored you bank at about 45 deg or more unless you are sharing space with somebody else and must adjust to maintain position, or are sniffing out the best part. Most glider pilots don't bank steep enough.

Perhaps my question was unclear. I want to plot/calculate the graph of the radius versus $\phi$, the sink rate versus $\phi$, and the speed in a turn versus $\phi$.
Let me know if you’d like help with calculating or plotting these!

Think of min sink rate as being a parameter associated with one particular angle-of-attack. It's the angle-of-attack that maximizes (Cl^3)/(Cd^2). The airspeed associated with this angle-of-attack will vary according to the square root of the load factor.  So will the sink rate.  This is an an approximation because it doesn't take it to account that the "relative wind" is actually curved in circling flight, and the fuselage doesn't "bend" to conform, so you are imposing a smaller "decalage" angle between wing and tail, at least from the airflow's perspective, than would exist in linear flight.

(At least in case of fixed horizontal tail rather than all-moving tailplane.)  Compensate as needed w/ elevator, and that's why the stick is further aft in circling flight than at the same a-o-a in linear flight.  Which will have some theoretical effect on efficiency, sink rate, etc.  Another way to look at the same issue is to look at the "pitch damping" effect imposed by the aircraft's (nose-up) pitch rotation rate while circling.  So the aerodynamics of circling are not exactly the same (just with speed scaled up as per wing loading) as the aerodynamics of linear flight.

The distribution of updraft velocity across the thermal will affect your choice of bank angle but shouldn't affect the equations relating bank angle, airspeed, and sink rate relative to the airmass.  If you want to incorporate sink rate relative to the ground into your model, then yes you have to give some thought to the distribution of updraft velocity across the thermal.  I would say-- "good luck with that", sounds very complicated, not sure there is any widely accepted model of what this looks like in reality.

If you want to give more thought toward aerodynamic effects related specifically to circling flight (and I'm not saying you really should) then see https://wingsandwheels.com/blog/post/thermalling-in-a-slight-slip, and search aviation stack exchange for some answers I've written referencing Holighaus.

Does this answer your question?

You have probably missed the right answers so far.
Minimum sink speed is indeed achieved at the maximum of c$_L$³/c$_D$², as proven in this answer. Note that for a very high aspect ratio wing this polar point might be unachievable because it requires a rather high maximum lift coefficient.
Now for the velocity: When banking, only part of the lift force can counteract gravity because the remainder is needed to provide the centripetal force needed for circling. As you write, flight speed needs to go up in proportion to the inverse of the cosine of the bank angle. Start with the simple connection between angular speed $\Omega$ and flight speed $v$:
$$v = \Omega \cdot R$$
with the radius of the turn $R$:
$$R = \frac{2\cdot m\cdot\frac{n_z}{\sqrt{n_z^2-cos\gamma}}}{\rho\cdot c_L\cdot S}$$
the angular speed $\Omega$:
$$\Omega = \sqrt{\frac{(n_z^2-cos\gamma)\cdot g\cdot c_L\cdot\rho\cdot S}{2\cdot m\cdot n_z}}$$
and use the lift coefficient at minimum energy loss when replacing $R$ in the equation above:
$$c_L = \sqrt{3\cdot c_{D0}\cdot\pi\cdot AR\cdot\epsilon}$$
and the load factor $n_z$:
$$n_z = \frac{1}{cos\Phi}$$
Now we have all in place to calculate the flight speed at the given bank angle $\Phi$:
$$v = \sqrt{\frac{\left(\left(\frac{1}{cos\Phi}\right)^2-cos\gamma\right)\cdot g\cdot \sqrt{3\cdot c_{D0}\cdot\pi\cdot AR\cdot \epsilon}\cdot\rho\cdot S}{2\cdot m\cdot \frac{1}{cos\Phi}}} \cdot \frac{2\cdot m\cdot\frac{\frac{1}{cos\Phi}}{\sqrt{\left(\frac{1}{cos\Phi}\right)^2-cos\gamma}}}{\rho\cdot \sqrt{3\cdot c_{D0}\cdot\pi\cdot AR\cdot \epsilon}\cdot S}$$
cancelling terms allows us to further simplify the speed equation to
$$v = \sqrt{\frac{2\cdot m\cdot g\cdot\frac{1}{cos\Phi}}{\rho\cdot \sqrt{3\cdot c_{D0}\cdot\pi\cdot AR\cdot \epsilon}\cdot S}}$$
which is basically the definition equation of the lift coefficient written in an odd form and for the minimum sink condition. The corresponding sink speed $v_z$ is now:
$$v_z = v\cdot\frac{c_D}{c_L} = \sqrt{\frac{2\cdot m\cdot g\cdot \frac{1}{cos\Phi}}{\rho\cdot S\cdot\sqrt{3\cdot c_{D0}\cdot\pi\cdot AR\cdot\epsilon}}} \cdot\frac{4\cdot c_{D0}}{\sqrt{3\cdot c_{D0}\cdot\pi\cdot AR\cdot\epsilon}}$$
$$= \sqrt{\frac{32\cdot\sqrt{c_{D0}}\cdot m\cdot g}{cos\Phi\cdot\rho\cdot S\cdot\sqrt{(3\cdot\pi\cdot AR\cdot\epsilon)^3}}}$$
The results for an ASK-23 are plotted below:

Graph of speed, radius, load factor and sink speed in SI units. Since the z axis is oriented downwards, a sink speed is positive.
Nomenclature:
$c_L \:\:\:$ lift coefficient
$\epsilon \:\:\:\:\;$  Oswald factor
$g \:\:\:\:\;$  gravitational acceleration
$n_z \:\:\:$  load factor
$m \:\:\:\:$  airplane mass
$\pi \:\:\:\:\;$  3.14159…
$\Phi \:\:\:\:$  roll angle
$\rho \:\:\:\:\:$  air density
$R \:\:\:\;$  radius of the circle
$S \:\:\:\;$  wing area
$v \:\:\:\:\:$  flight speed
$v_z \:\:\:$  sink speed

Isn't the $c_L^³$/$c_d^2$ using the assumption of straight flight? in a curve $L=W$ does not hold anymore, so minimum sink speed doesn't occur at max $c_L^³$/$c_d^2$?

@Peter Kämpf Thank you for you answer. Thats exactly what I need. Can you maybe share a bit more of the intermediate steps to get the final formula, so I can fully understand what you did there?

@ROIMaison I dont know If this really changes in a turn. But a ascpect I also need to look at.

@ROIMaison In a turn lift is L = m·g·n$_z$ and the ratio stays the same. L = W is a simplification.


text:
How do you drain a fuel tank on a Cessna 172R?
Our 172 "R" model has 5 sump drains on each tank.  I leveled the airplane according to the POH, removed the sump drain located next to the inlet hose by the fuselage and drained the tank dry. Is this the proper way to drain a tank to "empty"?  Or should the airplane not be leveled in order to let any residual fuel move to the rear of the tank for drainage?  There is no published guidance that I can find to drain a tank to "empty".

@JohnK I have posted a similar pic here before, I fly the same model. There's 5 each wing and 2 under the fuselage.

I also reviewed the maintenance manual and studied Chapter 28 - Fuel Systems, more carefully.  Under "Fuel Quantity Calibration" it does say to remove all fuel drains on the wings and the from the fuel selector too.  However, it also says to level the airplane first and then tilt the nose up  by 2 degrees before adding fuel.  Previous guidance was to keep the airplane in a level attitude during fueling.  Maybe the tilting will make a difference.  I am going to try to recalibrate next week.


text:
Is there any combination of aircraft and airports where no alternate landing airport would be possible?
In the comments on this answer an interesting assertion was made, that you never have a situation in the US where there is no possible alternate airport.  Assuming that:

you're flying an aircraft that is normally flown for commercial reasons,
you're flying to an airport that normally allows flights to land,
you're flying from an airport in the US to another airport in the US,
and you're following the regulation that you have to have enough fuel to get to your destination, then fly to your alternate, then stay in the air for another 45 minutes

is there any combination of aircraft and airports where no alternate landing airport would be possible?

I can guess an A380 or concorde will have a more limited set of options

I'm not quite sure what you mean. If there's no alternate possible, you wouldn't be allowed to fly the plane there, right? So are you asking if there's some combination of plane and airport such that the plane could technically land at that airport but can't, in practice, fly there because there'd be no alternate?

I don't understand this question. Isn't it trivial to find a pair of airports at maximum range for any given aircraft and fuel-load?

@RedGrittyBrick all commercial aircraft must have enough fuel to get to the destination; miss an approach; then go land at the alternate, and still have enough fuel to circle for 30 minutes

@ratchet yes, but that's a regulation. The question seems to be asking if you can construct a hypothetical flight plan ignoring such regulations. That's why I don't understand what the question is asking.

Either that, or he's asking if there's an airport within the US that cannot legally be used because of said regulation (no alternate reachable). If there is, I'm guessing Alaska and some limited range aircraft.

Is this an AOE (Airport of Entry) for customs reasons type of question? If so, legally yes, you are supposed to land at an airport with customs first but many airports near international airports are also AOEs (at least that's how it is here in New England) so while there could be a place where you might be stuck I can't think of one off the top of my head. Anyway if you did divert in an emergency I would just suggest not deplaning until contacting CBP (Customs & Border Protection) first. [Through ATC even if necessary]

@ratchetfreak, that regulation was what I meant.  I'll edit the question to reflect that.

I'm not sure if the space shuttle was flown "for commercial reasons" but it comes to mind as an aircraft that wouldn't be able to make it to an alternate.

For larger aircraft, likely no. Worst case they can carry enough fuel to return to their origin. If range does became an issue (e.g. Atlanta -> Anchorage) you would just schedule a technical stop somewhere like Yellowknife. Alaska has a ton of small airfields so short-range aircraft would have many alternatives. Wether the alternate has any services is another question.
As the question asks about US airports only, mainland -> Hawaii would be a possibility. Hawaiian Air's 717 planes already need extra tanks in the fuselage to get to the mainland, so a return flight would be weather-dependent. If you take off from Seattle and several airports close for bad weather after you pass the no-return point your options are rather limited. If the weather is bad enough to close every airport in the islands then channeling Sullenberger won't help much. Unlikely any sane pilot would take off with that kind of forecast, and Hawaii's weather is usually pretty good.

Mainland-to-Hawaii is about the only (US) case I can think of where there may be no alternate possible (and even then only in rare situations where all the islands are hit by lousy weather - Hilo and Honolulu are about 180 miles apart which should be well within diversion range for something like runway fouling).


text:
Which aircraft require that the vertical position of the centre of gravity is checked?
We normally check the aircraft CG along the longitudinal axis and the lateral axis (sometimes referred to as the transverse axis) to ensure it is within limits. Until recently I was unaware of any aircraft that required a check and control of the CG along the vertical axis. None of the aircraft (B727, B747, DC-8, DC-9, L1011) that I have been involved in weight & balance detail for have required a vertical CG check. Recently, however, I have found that some B767 freighters do.
My question is: Are there aircraft other than some 767s that require a vertical CG check?
Edit in response to a comment: My purpose in asking the question was to learn how others might have handled a vertical CG check. You can see the method I used by going to http://terryliittschwager.com/WB/B767/load.html and searching the page for 'vertical'.
Additional edit in response to additional comments: Some comments have expressed the opinion that there is little or nothing that can be done about the vertical CG. In practice, the vertical CG of cargo (and thus the airplane) is easily affected by specifying which ULDs (unit load devices—the so-called "igloos") will be used for loading. ULDs come in various standard heights. 96", 64", and 45" are oft used heights but there are others, and an array of custom heights. A usually safe assumption is that the mean vertical CG across ULDs is no higher than half their height. In the case of the 767, the published weight & balance documentation gives tables for average CG heights of 42" and 36".

I would expect this more on low-wing cargo aircraft like the Boeing 747 Dreamlifter, Aero Spacelines Super Guppy, or A300-600ST Beluga. I can't find any SOP's for any of those aircraft to verify though.

Helicopters are sensitive to vertical CofG and in general, should have the CofG under the rotor mast and as low as possible.  This is to reduce the likelihood of dynamic rollover.  A higher CofG also reduces roll authority.  That said, I am not aware of any helicopter that includes it in the W&B calculations.

@Simon That's probably due to there being little you can do to adjust the vertical CG with cargo/fuel.

@KorvinStarmast There is actually a lot you can do, mainly not taking off. The lack of the ability to adjust it doesn't necessarily mean its not important to verify within limits. The reason it is probably omitted is because, most likely, in any loading configuration that is within the aircraft maximum weight limits would be inside the vertical CG limits given the cabin configuration.

@RonBeyer  You can't do anything substantial about the vertical CG location.  What you can do is not drag your skid or main mounts, laterally while taking off.

@KorvinStarmast You can't modify the vertical CG, but you can elect to not take-off in an unstable aircraft, and you need a way to verify that if it is applicable.

@RonBeyer I realize we are separating the fly turds from the pepper here, but if that aircraft is certified to fly by FAA or CAA or EASA, then the "it's an unstable aircraft" has already been addressed via the development and cert process, has it not?

It has a published/certified design envelope, yes, but that doesn't mean the operator hasn't loaded it incorrectly causing it to be unstable... For aircraft that have vertical CG limits that can be exceeded within MTOW, you need to be able to verify you haven't exceeded them, otherwise the aircraft will have controllability issues, like the 767 Freighter aircraft Terry mentions.

I found examples in the first few entries in google

@user6035379 What search string did you use?

@RonBeyer military aircraft (particularly carrier aircraft and those with external stores) are sensitive to vertical Cg on landing.  That is one of the reasons that they have certification limits for bring back of ordnance.  While you can't "modify" it, it is not uncommon to jettison heavy ordnance prior to landing and part of that gets the Cg in the right basket (but the bigger driver is landing load on the pylons...usually).

@RonBeyer I wrote the weight & balance software used by the Dreamlifter while Evergreen International Airlines was operating them for Boeing. During that time, at least, there was no vertical CG check required.

Since no one mention it I would propose Shuttle Carrier Aircraft
It is mentioned in page 4 of the Space Transportation System Cargo Abort and Recovery Operations: (pdf)

This requires not only removal of the payload but removal of Orbiter main engines, tires, landing gear and other components as well. In addition, the Orbiter Z axis (vertical) and X axis (longitudinal) center of gravity location must be within a limited envelope, as shown in Figure 1.

Related : https://aviation.stackexchange.com/a/23808/679

In many aircraft the vertical (Z axis) placement is of little impact on weight distribution.  However, in some aircraft it affects maneuverability.  So for an aircraft like a C-5 it is noticeable, yet a C-5 is not considered an aircraft where vertical CG would be a factor, as the nature of the operations are, well, cargo hauling.  In a helicopter, which tends to have a higher (Z-axis) CG with the top placement of the engine and transmission, the considerations are greater. It doesn't impact loading in a normal sense, but there is a noticeable impact in handling particularly when roll moments are involved.
Stated perhaps more simply, static stability is normally bounded by longitudinal and to a lesser degree latitudinal forces. However, a higher CG will impact dynamic stability where the consideration is not primarily limited to wing (rotor) and airfoil loading.
So the simplest answer is that it does make a difference, which is more than just measurable in some applications, but normally the difference is of small consequence and only a tertiary consideration.

This does not answer the question, and provides nothing other than opinion.

A common consideration of high vertical CGs is the effect of thrust changes on pitch in a fixed wing aircraft.

Why no one has mentioned the biplanes and other non-planar wing arrangements for which the z-coordinate of CoG may be more sensitive and thus affect handling and stability characteristics?

As Mongo mentioned, it is generally more important in taller aircraft. However, it must be noted that small aerobatic aircraft also have the operator check the vertical CG. A good example of this is the Extra 300. Look up a POH and check out the way CG is handled in this aircraft - it's really interesting and different than any other I have seen.

Ah, @Pugz, I forgot about the Extra 300.  Good example.

Seaplane designers need to take vertical CG into account when designing landing gear. The CG needs to be 10 degrees forward of the step in the hull, so a higher CG needs weight more forward on the landing gear. It's not adjustable as part of operation, but it's necessary for design.

In that sense all airplanes with tricycle gear must take vertical CG into account. And they do during design, though the angle is usually taken about 15°. Otherwise, like seaplanes, they would have difficulty rotating on takeoff.


text:
Why do different sites list different lengths of the lower deck bulk cargo compartment for the A330-300?
From the following two links for Airbus A330-300, the length of "lower deck bulk cargo compartment" has two different values.
a) Air China lists the value as 511 cm
b) Airbus (see pg. 112) lists the value as 404 cm
Why?

The Airbus document says: NOTE: APPROXIMATE DIMENSIONS DEPENDING ON AIRCRAFT CONFIGURATION.

It is interesting they list the same usable volume though.

fooot, you are right, in Airbus link we are referring about, there is note which declares the dimensions are approximate. But, about "..they list the same usable volume though.", considering only the two links called above, I can see that volumes value is provided only by airchinacargo.com/en/index.php?section=0-0149-0155-0176-0266 and it's not provided by by http://www.aircraft.airbus.com/fileadmin/media_gallery/files/tech_data/AC/Airbus-AC_A330-Dec16.pdf

Some airlines have different specs. I can't comment on the examples you've given but one example I have encountered is on the A340-300.
The airline I worked for had conventional LD3 loading where the cans were loaded in pairs but for a while we had an A340 leased in from Swiss or Sabena (cant really remember which as it was more than 10 years ago) which had an additional LD3 position on the centreline of the aircraft in the space that would usually be available for bulk loading.
On thing which could affect the size of the bulk compartment is the location and number of toilets on the passenger deck. The plumbing usually lives under the floor.
Note the airbus site mentions that dimensions are approximate.

Thank you Anilv. So, the conclusion I have to understand is that, in dependance of the Customer the in the same Aircraft model, the size of the empty space usable for cargo hold could be often variated often by Manufacturer? But in that case everytime the engineering offices have to approve a modification in design that Hypoteticaly could affect the structure?

The empty space usable for cargo is constant. It is only how the space is used. Eg is it loaded as bulk (loose items) or in a ULD.


text:
What are good study guides/practice tests for the airframe maintenance certificate?
I am approaching my 18 month mark on the job. Wanting to get my A license first. I have done all kinds of airframe work in this time period. Wondering if anyone has suggestions for the names of the good practice tests books for general and airframe. I have the FAA books on all 3 subjects just no study guides or practice tests. Any suggestions?

I used the Dale Crane series of study guides from ASA years ago, available from either of those two links as well as many other places.
Here is the specific study guide for Airframe, but you will also want the General study guide. These are for written test prep, but might also include Oral and Practical (O&P) test prep.

Image from mypilotstore.com product listing
There are also other options out there. Gleim offers an AMT test prep course, as does ASA in their AMT Prepware, but I don't know that I have any experience with either one. I definitely used one of the digital test prep packages out there, but I can't remember which one. I do like the digital tests, especially since that is probably how you will be taking the writtens anyway.
I also studied the Jeppesen textbooks in addition to the FAA textbooks you mentioned.
You will also want to study the Practical Test Standards (PTS) for your O&P test prep.


text:
How does a crosswind affect an aircraft's speed relative to the ground?
An aircraft heading west at an airspeed of 100 km/h has a crosswind blowing from the south at 100 km/h. What will be the aircraft's speed relative to the ground?
The correct answer is 141km/h. Can anyone explain and give the formula?
What is it? I know it but I need the formula with which it is calculated.
I found Pythagorean theorem for this (but since the directions of heading and wind are not the same result is not same here, so it must be another formula).

so it must be another formula why?  It's just Pythagoras.   Sqrt of a2 + b2.  On the link you provided, just look at the 3rd example.

I'm voting to close this question as off-topic because it is more appropriate for http://physics.stackexchange.com

@RonBeyer I would have thought Math.SE would be better,.  There are no physics.  It's just a triangle of vectors.

@Simon These are pretty textbook for 100-level physics courses, and pretty much confirmed by the OP's follow-up which involves one of the basic acceleration formulas.

@RonBeyer True.  I voted with you anyway.

Simple Answer
The Pythagorean Theorem does give the correct answer as the square root to the whole number of $100^2 + 100^2$ is indeed 141. This works only if the wind is blowing at right angles to your compass direction.
More trigonometric function-based method
Another way to get to 141 is to divide 100 by $\sin 45^{\circ}$ or $\cos 45^{\circ}$ because the triangle has two equal legs. (Your airspeed and wind speed are equal at 100 knots.) Therefore, the angles opposite them must be equal.
Now, since it is a right triangle and the sum of angles in a triangle is 180, the other two angles’ sum must be 90. Since the angles are equal, $\frac{90}{2} = 45$. The sine of an angle is the ratio of the side opposite the angle to the hypotenuse (your groundspeed). Doing a little rearranging of the terms, it comes out that the groundspeed is 100 divided by the sine of 45.
$$\begin{align}
\sin 45^{\circ} & = \frac{\mathrm{crosswind}}{\mathrm{groundspeed}} \\
\\
\mathrm{groundspeed} \cdot \sin 45^{\circ} &= \mathrm{crosswind} \\
\\
\mathrm{groundspeed} &= \frac{\mathrm{crosswind}}{\sin 45^{\circ}}
\end{align}$$
Now, for the other angles created by the wind-heading combination, you would have to get the different components together and use either the Law of Sines or Cosines, depending on what components you have.

I would expand on where you got the 45 from, it may not be apparent to the OP that it is the actual direction of travel, especially when 45 does not match any compass heading but is in Cartesian coordinates. On top of that using Sine or Cosine is only valid at 45 degrees because the result is the same. Your answer was correct until you added that part.

@RonBeyer Added more explanation, is it better?

A little bit but the explanation breaks down at different speeds, imagine if the next question said 100km/hr for the airplane, and 20km/hr for the wind speed. How do you derive the angle then? Or if the airplane heading was 260° and the wind heading 010°?

divide 100 by sin45° or cos45° this only true because the angle is 45°, would not be true otherwise, as the law of sines gives a sine, not a cosine. personally I would remove the reference to the cosine to avoid confusion.


text:
What is the most accurate geometric plane for referencing terrestrial flight attitude?
I have found various definitions for attitude.

Merriam Webster: inclination of the three principal axes of an airplane in flight to the relative wind.
Wikipedia - Attitude Indicator: the orientation of the aircraft relative to Earth's horizon.
In my words, based on my current understanding: the angular orientation of the longitudinal and lateral axes relative to a 2D geometric plane, which is orthogonal to the Earth's average curvature at the point on the surface below the aircraft.

What is the correct reference plane for flight attitude in regards to terrestrial aerodynamic flight?

Related discussion: Why is pitch measured with respect to the horizon and not the ground?

Definition 2 says nothing at all about pointing the aircraft at the horizon.

For definition 2, the attitude indicator is not referenced to the horizon line, but to the horizontal plane. Increasing altitude doesn't change pitch on a gyroscopic AI. Similarly bank angle is not measured relative to the horizon line sideway, but to the same horizontal plane. The latter is a plane parallel to the surface of the geoid).

@mins what you're saying makes sense.  It could still be a horizontal plane that is tangential to the Earths average curvature at the point below the aircraft; it would just run beneath the segment of the Earth that is visible above the horizon.

@MortensenAviation, the “true horizon”, in the sense where you see ground/sky boundary, isn't even a plane, so it can't be used as reference.

@JanHudec sure it can in the same way that the flat spot left when you chop off a slice of apple is a plane.  The plane created by the 360 horizon goes beneath the surface of the Earth but would nonetheless be tangential to the average curve of the Earth's surface beneath the aircraft.  Whoever pointed out that the lateral axis doesn't need to point at the horizon to properly indicate 0 made a good point.  Of course this is from a single observer's perspective.

@MortensenAviation, well, that plane is actually parallel to the horizontal (almost, because geoid is not exactly a sphere). But if you are looking at the horizon, you are looking from your position and that plane does not contain your position.

@JanHudec Yep, you're right!  I replied to that last one before going home last night and that dawned on me during my commute; If sliced off at the true horizon, it would look like a pringle (not literally that steep but we get it) instead of a flat circle because of the geoid shape.  So, that is out, but then I thought the weight vector would accomplish this as well, and then I read your answer this morning.

Attitude is rotation of the body frame of reference (its principal axes, whatever it means) relative to a chosen frame of reference.
For aircraft:

The body frame of reference is the longitudinal, lateral and vertical axes, where the longitudinal-vertical plane is the plane of symmetry and longitudinal axes is chosen to point generally forward, along cabin floor if there is such a thing.
The chosen outside frame of reference is the local horizontal, which is plane perpendicular to the local weight¹ vector, and in that plane the main axis is chosen either as projection of either local magnetic inductance vector (magnetic heading) or Earth rotational axis (true heading).

Regarding “true horizon”, in the sense of where the you see the ground/sky boundary, that can't be used for defining attitude at all, because it is not a plane. The line connecting you to the “true horizon” ahead and the line connecting you to the “true horizon” behind is not the same line, so it can't be used as axis.
Instead, horizon is defined in terms of horizontal, the plane orthogonal to the weight vector (and tangent to the Geoid, the ‘idealized’ sea level).
To fly level, your velocity vector must point along the local horizontal. In most aircraft it means your pitch is slightly positive. This is because most aircraft are designed to never need to fly at negative pitch² and as the angle of attack for level flight depends on weight and speed, most of the time your pitch will be slightly positive.

¹ Not “gravity”, because it centrifugal force due to Earth rotation is included. Actually, the instruments measure the value including the centrifugal force due to the movement of the aircraft, which might be slightly different from centrifugal force on stationary object. The immediate measurement also includes the inertial force due to acceleration (the forces can't be distinguished), but this is eliminated by averaging, because the average acceleration (over several minutes) is known to be zero or close enough for practical purposes.
² There are some exceptions that do normally fly slightly nose-down, like B-52, which does it because its unusual gear does not allow it to rotate on take-off, or DHC-6 Twin Otter, which apparently does it to provide better view during slow short take-offs and landings.

There are a few definitions because there are a few ways to look at it.
The most general thing to say would be something along the lines of,

An object's angles in pitch, roll and yaw from a set of chosen zero planes

It boils down to what you want to use as your point(s) of reference. Attitude indicators use the horizon and if you really want to talk about it even space ships have gimbals that measure attitude relative to a plane. Your definition assumes only attitudes of aircraft flying over the earth (using the surface as a reference plane) which is valid for most cases but not all.
Furthermore, attitude indicators are referenced to the horizon because it is an easy to find and generally a universal level plane with respect to the Earth's surface. In aircraft flight we care about the Earth's surface because that is where we want to get back to and subsequently not smack into in the process, so its convenient to reference to its plane. Furthermore at high altitudes the horizon is still the horizon; it simply becomes a space/Earth horizon as opposed to a sky/ground horizon.
Here is the view out of the SR-71's cockpit (@73,000ft, this is also the highest-flying non-experimental plane ever made) to illustrate the point.

(source)
To extend the frame of reference explanation, we can look at the Apollo rockets FDAI, which is a nice example of a 3-axis attitude indicator that was referenced to multiple planes instead of a single horizon. In this case the flight attitude was simply the angle from a chosen set of zero planes.

The first one is called "Inertial Stellar". That means that the reference is a 3D system of coordinates which is fix relative to the stars. That does not mean that there's only one system of reference. In fact, there is a system of reference for almost every phase of the mission. Some are reused during the flight and. in order to be accessed multiple times, they are stored in the computer as "REFSMMAT", which stands for REference For Stable Member Matrix.

-- EDIT --
Since the root question has changed somewhat ill add to my answer. Generally speaking airplanes are oriented to the gravitational force below which is taken to be the "vertical". The vertical is often taken to be the tangent of the ground beneath it but since the ground may also not be level this is a generalization. There is a bit of real world discussion involved here since attitude indicators, the point of reference for an aircrafts attitude may not always be perfectly level to the ground. A pilot is mainly concerned with setting his 0 degree point with a pitch that will yield flight that does not increase or decrease in altitude. This is called "straight and level flight". In reality, due to the design of aircrafts this may in fact be a few degrees of pitch away from true 0 on the reference plane. Power settings will also effect this angle.

Yes, the horizon's definition does not change, but the higher one flies, the smaller the angle between the weight vector and the the longitudinal axis, when pointed at the true horizon, becomes due to Earth's curvature.

Should yaw be considered in attitude?  Going by a "chosen zero plane" a 180 degree turn would be a change in yaw.  IMHO yaw is a separate consideration.  It is utilized differently than pitch and roll indications and is referenced to relative wind for the purpose of maintaining coordinated flight or performing slips. I suppose you could reference it to something else, but I can't think of why one would want to.

I just added an edit, generally yaw is considered differently but in its purest form attitude has reference to all axis in 3 Space. While aircraft attitude is often only looked at from a pitch/roll perspective, flight attitude may extend to other scenarios.

I apologize, I didn't initially write the question to be specific to aerodynamic flight.  Space flight is different, and I am not an expert on that, but I would imagine that a yaw reference would be used for 3 dimensional navigation in the way that airplanes use a heading indicator. So yeah, it actually would make sense for it to reflect a 180 degree turn as yaw in that scenario.

Yaw would come into play potentially in an INS unit (which many large planes have) as well as in a flight data recorder and the like. But you are correct that commonly airplanes do not display it on a traditional attitude indicator. Modern glass cockpits do integrate a slip/skid display into the attitude section of the screen.


text:
Did any WWII aircraft only use a turbocharger but not a supercharger?
In doing some basic research, I have concluded that all WW II aircraft equipped with turbochargers also used mechanical driven centrifugal superchargers.
These include the P-38, P-47, B-17, B-24, B-29, etc.
Is this correct, and are there any exceptions where the engine lacked a mechanical supercharger and only used the turbocharger for boost?

Original poster, if you are able to come up with a good answer to this question, please share it with us.

This website gives some hints as to why a gear-driven supercharger would normally be used in addition to a turbosupercharger: http://rwebs.net/avhistory/opsman/geturbo/geturbo.htm

Gear/belt driven supercharger runs at constant speed. I don't think the compressor works well at all altitudes if it has a fixed speed. For a exhaust-driven turbo charger at least you can adjust the waste gate valve.

As is suggested in the original question, the answer appears to be "no".  This website gives some hints as to why a gear-driven supercharger would normally be used in addition to a turbosupercharger: http://www.rwebs.net/avhistory/opsman/geturbo/geturbo.htm
(Though the link may raise more questions than it answers: it doesn't make it obvious why a turbosupercharger usually injects air upstream of the carburetor, or why a gear-driven supercharger usually injects air downstream of the carburetor, or why only the latter prevents issues with valve cooling.)


text:
What is this second smaller runway next to London City Airport?
I was looking at London City Airport (EGLC) on Google Maps today when I noticed the remains of a very small runway marked 10/28 just across the water from City Airport's main runway. It can be seen here.
To my surprise I couldn't find any mention of this small runway after searching online and looking into the history of the airport. Using the Google Earth historical imagery feature the earliest image I could find that included the runway was 2008, so I assume it was built some time between 2006 and 2008.
Can anybody tell me what this short runway is for?

God! Nice shot! I admire you being able to spot things like this! :>

It hasn't got the usual X for a closed runway?

Good Spot! That was a temporary runway for the Red Bull Air Race:
https://www.forgottenairfields.com/airfield-london-redbull-airstrip-1429.html
You can see "RB" at the eastern end on your satellite imagery

According to the English wiki the River Thames was used as a race location for the 6th round on July 29 in 2007 and for the 5th round on August 2 and 3 in 2008.

12 years on and that's looking like it's in pretty rough shape already.

The page says London City was closed during the race anyway, which makes me wonder why they didn't simply use it.

@JanHudec Maybe because of LCY's rule: steep approach and steep prices ;-)


text:
Why pets are allowed only on select flights?
If the baggage compartment is always pressurized, why pets can be brought only to select flights and not all flights?

The USDOT has a list off things related to this matter here but there are many factors.

International Travel: Since flights often go between countries that also means they cross jurisdictions. Many countries have varying laws on importing and exporting different animals. As such airlines may not always allow you to carry animals since it may not be legal.
Contamination: Animals can carry a wide variety of disease and other things. An airline my chose to not carry animals to avoid any potential issues. Even if in the cargo hold it can not be guaranteed that the animals fur wont come in contact with another passengers baggage. If that passenger had an allergy to that type of fur they may later have an issue.
Food: Placing an animal in the baggage hold prevents anyone from effectively feeding the animal which the DOT does seem to have some regulations on for longer flights.
Temperature: Although it is pressurized the baggage compartment is by no means warm. This prevents a potentially dangerous situation for animals as well.

As for bringing an animal on the plane (aside from service animals which are not considered animals). It would seem that the FAA leaves this specific matter up to the airlines. According to this document when it comes to

What are the FAA rules regarding traveling with pets in the passenger cabin?
The Federal Aviation Administration (FAA) allows each airline to decide if they will allow you to travel with your pet in the passenger cabin. If an airline does allow you to bring your pet into the cabin, we consider your pet container to be carry-on baggage and you must follow all carry on baggage rules (14 CFR part 121, section 121.589):

There is also air circulation, not all holds are designed to circulate the air enough to keep the animal healthy.

There are also ramp concerns: live animals generally can't be left out on the ramp in the heat (or cold) while baggage is loaded/unloaded, and need special handling once they're off the aircraft (they're not just dumped on the baggage carousel). Ground personnel need to know there are animals onboard so they can be handled appropriately (last on, first off, and taken to the appropriate area of the airport on arrival to meet their owners).


text:
What is the biggest airplane that does not need a paved runway?
Since we don't need a control tower for instrument landings I was wondering what else that is thought to be needed could be skipped. I know some small general aviation is done on grass runways and I was wondering what is the biggest airplane that does not need a paved runway? Examples of manufacturer-approved usage or regular (e.g. daily or weekly) usage are preferred.

A couple of questions to help clarify.  Do you mean planes that are currently in production, or just historically?  And when you "doesn't need a runway", what do you mean?  Grass, dirt, aircraft carrier, water, rooftop?  Also, it might be best to differentiate between military and civilian transport just because they are such different missions...  Right now this question is...pretty broad.

I was thinking any thing in use or advertised as available for purchase making at least two landings a year on an unpaved runway. I was not thinking seaplanes. As I mentioned biggest that probably rules out both carrier and roof both of which however can also be argued to be paved which also rules them out, but I know what my next question is going to be: what is the largest fixed wing aircraft to land and take off from a building?

"we don't need a control tower for instrument landings" is somewhat misleading summary of the linked question. Especially the dubious phrase "instrument landings".

Does ice qualify as "not paved"?

Does a metal runway count as paved?

@Compro01, cool link, not paved.

@JonathanWalters, cool link, I don't know. I would have said yes before reading that article but now I just don't know. I think that means I learned something.

God this sounds like OP wants to smuggle cocaine or something!

@k1308517, nope. No practical application. Just Curiosity.

Related: http://aviation.stackexchange.com/questions/19313/how-heavy-is-too-heavy-to-land-on-a-grass-strip

The space shuttle. Any aircraft can land on an unpaved runway. In fact, a dirt runway makes the landing easier. Hard runways are used because they are easier to maintain, not because it makes the landing easier.

http://corporatejetinvestor.com/custom/PrivatAir%20Boeing%20737%20antarctica.jpg

@TylerDurden Good point regarding dirt runways, but some of the examples already listed in the answers are quite a bit larger than the Space Shuttle (assuming we're referring to just the orbiter and not the whole stack... which seems like a reasonable assumption, since the whole stack wasn't really capable of landing on anything.)

Military transports are designed to be operated from unpaved runways- by extension, any of these converted to civilian use can operate from unpaved runways.
For example, the Antonov Airlines operates a number of Antonov An-124 'Ruslan', which can and do operate from unpaved runways regularly. The Volga-Dnepr group specifically states:

Multi-leg landing gears equipped with 24 wheels allow to operate the aircraft on unpaved runways ...

Now only if somebody could find a definitive reference for An-225. Its 32 wheels certainly look like it should be able as well.

@JanHudec I'm not sure if it could. An-124 was developed as a military transport, while the 225 was designed to transport the Soviet shuttle. Anyway, I dont think the manufacturer or operator recommends its operation from unpaved strips.

Well, it probably wasn't design requirement for An-225, but the aircraft is a scaled-up version of An-124 and the gear is most likely the same, just with two extra legs on each side. If it has the same (low) pressure, it should be usable on unpaved strips. In the end the question is probably whether the operator (Antonov Airlines) would risk it if somebody came with the money and a good reason, or if they even ever did.

The H-4 Hercules (Spruce Goose) should easily win this contest.  That plane was ridiculously huge and, in fact, couldn't use a runway, rather it was a "flying boat" that took off from the the water.  So I'd say it's the one...
Granted, if you mean a grass or gravel strip aeroalias is probably correct.  The only other major planes that I know were designed to land on gravel were the early 737 and the 727.  They both had special landing gear guards that keeps the front gear from kicking gravel into the engines and both have specific instructions for preparing the landing gear for touch down on gravel.
Here's the guard, if you were curious, on a 737:

Source https://www.flickr.com/photos/capnmikesphotos/14891693505, Author: Mike Pearson

At 180t “loaded” weight (MTOW probably wasn't properly calculated as it was just a prototype) H-4 is tiny compared to An-124's 405t, though it had longer wing span.

Also, most the military transports were designed for grass or gravel strips, including C-5 and C-17, plus most civilian transports designed in the Soviet Union were too (and are often used for it, because airports in Siberia are generally unpaved).

Another question would be how these things retract.

Gravelkits for MD-90s and BAe-146s are also available....

wow the guard is so close to the ground surface.

@Loong Never said they were required, just helpful.

Seems to me "big" is a matter of size rather than weight, so I agree - Spruce Goose!

It's not quite as big as the Antonov An-124, but honorable mention would have to go to the Lockheed LC-130.  It's designed to resupply scientific and military operations in polar regions, and so it has a dual wheel-and-ski landing gear setup that allows it to take off from and land on snow and ice.

(Image source:  Wikimedia Commons)

They also fly the C-5 Galaxy there. It has about 5.4x the MTOW of the C-130.

@reirab:  But does it have skis?

These oldies seem worth mentioning...
If you loosen your definition of "airplane" to "the longest class of flying machine and the largest airship by envelope volume" you could think of a Hindenburg-class airship that was in regular commercial service.
They landed in  fields
They're more comparable in size to buildings or very large boats, rather than other airplanes

Hindenburg in green compared to
  Blue: The Pentagon building,
  Pink: Queen Mary 2, ocean liner
  Yellow: USS Enterprise, supercarrier
  Dark blue: Yamato, WWII Japanese warship
  Grey: Empire State Building
  Red: Mont, a supertanker

Compared to other airplanes (Hindenburg in orange):

Or the Spruce Goose (* Not acutal Spruce). (Just noticed it mentioned in another answer, after posting this)

Originally designated HK-1 for the first aircraft built by Hughes-Kaiser, the giant was re-designated H-4 when Henry Kaiser withdrew from the project in 1944. Nevertheless, the press insisted on calling it the “Spruce Goose” despite the fact that the plane is made almost entirely of birch.

Apparently it is "the largest flying boat ever built and has the largest wingspan of any aircraft in history" at 320 ft 11 in (97.54 m). Not strictly a "regularly used" airplane, but

Hughes retained a full crew to maintain the mammoth plane in a climate-controlled hangar up until his death in 1976.

Going off on another direction if ice counts as "unpaved", a Boeing 757 was landed in Antarctica in November 2015, the first commercial airliner to do that.

Source is from the link above.

While this might be the first time an airliner operated by an airline had landed there, it was not the first 757 to land there.

As @reirab notes, plenty of non-airline operated "airliner type" aircraft have been used in Antarctica. (I think the Australians currently use chartered A319s, for example.) Even restricting it only to commercial airlines, the US Navy hired a Pan Am Boeing 377 to fly to McMurdo as early as 1957(!). It became something of a press sensation as it had two female stewardesses on board...

Another option might be the Soviet Ekranoplan: these are aircraft that use the ground effect to fly just off a surface of water.
For example we might have the Lun-class (73m):

or the experimental Caspian Sea Monster (92m):

Sadly neither of these are flying any longer.
(source: Wikipedia Ground effect vehicle and Lun-class ekranoplan)

This kind of stretches the borders of "flying" somewhat.

Lots of science fiction spaceships look tame in comparison - why haven't I seen these crazy things before?

My gosh!  can that first one even fly?  That makes even the most weird aircraft pale in comparison.

@dalearn They used to ask that about helicopters as well. :-)

"The MD-160, dubbed the "Caspian Sea Monster" by US Intelligence services, was indeed one of a kind. This massive 550-ton seaplane measured 240 feet long and 63 feet tall with a 144 foot wingspan—that's longer than the Spruce Goose and bigger than many modern commercial airliners. It was capable of carrying up to 137 tons (270,000 pounds) of troops and equipment—including as many as six nuclear missiles—at speeds up to 350 MPH as far as 1,080 nmi—albeit only 16 feet off the surface of the water."  https://gizmodo.com/this-caspian-sea-monster-was-a-giant-soviet-spruce-go-1456423681

authors: Vladimir F Героям слава


text:
Has anyone considered putting a hydrofoil on a sea plane?
One of the main problems with water landings is that you need a bigger engine to counter the various drags caused by the pontoons. The specific case of "run[ing] across our own wake to be able to get up on the step on glassy water" brings some interesting thoughts.

[B]asically a floatplane has to break the suction of the water - kind of like pulling your boot out of mud - and if the water isn't moving at all then it has to do that all by itself. If there are some waves, or a wake, or any kind of discontinuity in the water, then that gives the aircraft the momentary break it needs to overcome that initial suction. "The step" in this context refers to the step-up shape you see partway down the float (or hull); when you're "on the step" then you're using the part of the float forward of that step to hydroplane.

It is well known that water's significantly higher density makes high speeds difficult. A solution to that problem is the hydrofoil - a small underwater wing that raises some or all of a boat's hull out of water to reduce friction and increase speed.
So here is my dumb question: would placing a hydrofoil under a pontoon save fuel on takeoff? And how likely would the increased drag during flight cancel that advantage on a typical flight?

A hydrofoil is a wing that would only really work while on water. All sea planes already have wings that work while either on water or flying. I find it very hard to imagine a situation where adding the lift to the wings you already need anyway wouldn't be more efficient than adding another set of wings that only work part of the time. Especially since sea planes spend much more time flying than they do taking off, which is the only part where the hydrofoils would help. (And only at a specific short part of the take off too.)

@VilleNiemi it's all about drag. There's a ton of power required to get up on the step and into planing mode, and anything that could be done to reduce that requirement would aid in overall efficiency. Engines are heavy; some of that could get turned back into payload.

@egid actually it is not all about drag. You have to also consider mass, specifically the hydrofoil needs to be fairly robust and hence heavy or you risk fairly serious accidents when you run into waves.

@egid  Yes, but there may only be a very brief window where it helps you break onto the step (ie: start hydroplaning). My instincts tell me that it would just become a massive sea anchor for speeds above that, and dragging a wing through the water all the way to takeoff speed seems like a hundred problems in one.  Consider just the effective AoI and lift differential between the air wing and the water wing, especially at speed, and I'd be surprised if such a contraption didn't cause the plane to jerk and pitch uncontrollably everywhere in the envelope.  Seems a highly unstable arrangement.

It seems like a fundamental problem would be that the hydrofoil would create extra drag - a LOT of extra drag. So it would make it more difficult to get up to flight speed. Seems like it would do more harm than good. Seems like @Terry's answer indicated that the suction problem is really only a problem in a dead calm anyway

Why would the foil create more drag? These are devices that can lift many-ton ships out of the water. The size required for an airplane isn't going to be crazy.

@egid, some drag during flight would be inevitable firstly as there would be lift induced drag which should not be much of a problem as lift is desired any way, then there would be multiwing turbulence and general turbulence, inertial drag as it has mass and so on. The  hope is that since it is a small wing designed to create lift in the denser media of water that the total drag increase would result in less fuel usage than would be gained by reduced drag during takeoff would save fuel.

Drag and weight both count against fuel efficiency. I'd argue that foils probably add less drag than floats, and less weight than a more powerful engine. The net efficiency gain would be positive. The downside is harder control, significantly deeper displacement, and probably more maintenance.

@egid I guess what I'm saying is you'd have more drag (pontoons plus hydrofoil) until you gained enough speed to lift the pontoons out of the water. So it would depend on how much speed would be necessary to do that. If it takes 60 kts to lift the plane out of the water and 80 kts to fly then it seems you'd expend more energy getting to 60 than you would save in the next 20.

@TomMcW I think y'all are grossly overestimating the amount of speed required for hydrofoils to work. Wikipedia suggests cavitation starts causing issues at 60kts; you can now buy a kayak called the Flyak that takes off at 10 km/h,  and the US Navy tests someone linked below suggest that the research was productive, only being canceled due to a lack of interest in seaplanes in general.

@egid Just watched the video slebetman linked. The narrator says that they did get improved efficiency with a hydrofoil and more with skis, so it would appear that I am wrong.

A hydrofoil could lift the aircraft body out of the water at a low speed (e.g. 20kn) and this would significantly reduce drag during the rest of the takeoff run. The hydrofoil could be surprisingly small and could retract like landing gear once airborne. Even sailboats can exploit hydrofoils.

What I see as a problem is how to get the hydrofoil to pierce into water when the plane is lending and in a stable way. Too many things can go wrong during landing, e.g. strength required to withstand the impact, how fast the plane should "crash" into the water to prevent bouncing back from the water surface, and how to control the angle of attack (or decouple it from the AoA of the plane ) so that it doesn't shoot the plane back to sky. Without the hydrofoil controlled by a computer it's doesn't seem possible to achieve safe lending.

Yes. I mean, sort of. Nobody's ever built a production hydrofoilplane.
It's not a bad idea, but seaplanes are a relatively niche product these days and there doesn't seem to be a lot of innovation in the field. Back when there was, there was innovation everywhere so the idea of hydrofoils didn't stick.
The Convair F2Y Sea Dart, a 1950s prototype jet-powered fighter, used two 'hydro-skis' for takeoff and landing. It was more or less awful compared to the crop of land-based aircraft coming along and after a prototype disintegrated they (probably wisely) chose to shut the program down.

There are some other prototype aircraft that used actual hydrofoils that I found after some quick googling. A company called Lisa Airplanes seems to have an LSA - the Akoya - that's under development using tech similar to the Sea Dart.
The 1929 Piaggio P.7 used hydrofoils, and was intended to compete for the Schneider Trophy, but never made it airborne. Interestingly enough it had a screw (marine propeller) at the tail and a proper aviation propeller up front, and the pilot would've had to do some juggling between the two.

It's likely there were others, but I'm running low on sleep.

Found several interesting things googling around. The one that most intrigued me was this: https://www.youtube.com/watch?v=yolgS1bn7P8

It looks like the Piaggio P.7 was designed for the main hull to actually touch the water, rather than using separate floats. This seems worth highlighting.

Same is true of all the other examples. I don't think I've seen a floatplane with foils.

More on the Pc-7 (soon funded by Fiat and renamed Piaggio), but in French, and the original article by Giovanni Pegna, in Italian.

@mins You could translate it for us. ;-)

@TomMcW: No problem for the French article using Google Translate. Unfortunately the Italian original article is too voluminous for the tool, but there are many graphics that are interesting.

@mins I just look at the pictures anyway

That P7 looks lke it's have severe problems with the propellor. If the drawing is accurate it'd sit in the water, meaning the pilot can only spin it up once the screw has pushed the aircraft to enough speed to lift the fuselage onto the hydrofoils. And I don't even want to think about the corrosion problems that'd cause.

Also, this thing is struggling toward production: https://en.wikipedia.org/wiki/LISA_Akoya

It could make sense for a seaplane/amphibious to use a retract or semi-retractable hydrofoil (or 2 or 3) instead of the specialized fast waterborne planing hull.
 A simple barge-like hull (more aerodynamic, lighter/less robust since it's not hitting the water at 100+kts) would suffice up to maybe 15 kts. Much beyond that and the foil is starting to lift it. By 25, the hull is almost completely out of the water, clean out at 30+.
Starting at about 80, it's an ekranoplane, up to take-off speed.
The inflatable hydrofoils also are promising.
See the tests of a Goose with skis and with foils

https://en.wikipedia.org/wiki/Fairchild_C-123_Provider

"Pantobase" ski allows rough ground, or snow/ice, or water operation, if the plane has appropriate controls and can float. As soon as it gets above ~30kts, it starts lifting, and the hull is effectively out of the water at anything above 50
Lockheed looked at C-130 seaplane adaptation, floats, boat hull, and skis were examined
https://www.g2mil.com/c130seaplane.htm

Lockheed (Secretprojects forum has an excellent thread)

Aeronautical engineer David Thurston experimented with hydrofoils on at least one of his designs, I think it was the Colonial Skimmer. He stated in one of his books, I think it was Design for Flying that water is about 300 times the density of air, so a hydrofoil of very small size could lift an amphibious hull out of the water early in the takeoff run and significantly reduce overall drag. It also helped in handling higher waves, as the hydrofoil was on strut some distance under the hull, and the strut and foil could slice through waves with less impact than a hull.
I have a Phantom ultralight aircraft, I would like to experiment with inflatable floats with retractable hydrofoils. I believe the combination would be lower weight and drag than standard rigid floats. The step built in to the bottom of standard floats (required to break hull suction during takeoff) creates significant drag throughout flight. A retracting hydrofoil that is faired in flight should have less drag. And inflatable fabric floats are very light weight, and only have to handle water loads up to the point where the hydrofoil(s) lift them out of the water.
And if I build the entire assembly of poured concrete it will be very safe, as it will never get off the ground. Or water.

Actually, water is 800 times more dense than air. And hull suction needs to be broken when the hull starts planing, which is long before take-off.


text:
Why would a RAT be deployed at takeoff?
(Source)
Is that a Ram Air Turbine (RAT) on the port side below the wing? Why is it already deployed and spinning during takeoff?
This answer mentions a minimum airspeed at which RAT can be deployed.


text:
What does a blue light by the toilet sign mean?
When I was flying with Swiss we hit some turbulence. Of course, the seat belt light came on but above the bathroom sign, a blue light came on.

(Source)
Then after a while a second one came on. I'm just wondering if this has something to do with the turbulence or not. It was an Airbus A340 if that helps any.

Can you by any chance identify the plane you flew with? This might help if the said light is plane specific

It's the "Please only pee in a sitting position light" ;)

Alternatively, @SentryRaven, it's the "If you were standing when we hit turbulence, be a gentleman and clean up after yourself" light.

@FreeMan Or gentlewoman, don't discriminate here. And yes: I have seen things and know those exist... women who don't sit down...

@SentryRaven : It was time to distribute the travel johns.

You don't indicate which aircraft make and model you were flying in. I'm going to answer with some general information regarding Airbus aircraft, since that's the predominant manufacturer of aircraft currently in SwissAir's fleet. There may be subtleties from version to version.
Short answer: a blue light typically indicates a passenger has pressed the "call attendant" button above their seat. It's used to provide a central indication that assistance is required, and may help localise the row the passenger is seated in by only illuminating a blue light in some parts of the aircraft in closest proximity to the relevant row.

Longer answer
The panel with coloured lights to which you refer is the Area Call Panel (ACP). These panels are situated at either end of the cabin, and possibly in the mid cabin or at other locations depending on aircraft type and age.
The ACP typically has multiple colour indications, such as pink, blue, amber and green. Their meaning is as follows:

Pink: communication from cockpit. Normally steady, flashing if an emergency call is taking place (initiated in either direction: cockpit to cabin or cabin to cockpit).
Blue, steady: passenger call using at-seat attendant button in the passenger service unit (PSU -- overhead panel above each seat row). This light may only appear on particular ACPs, which enables the calls to be segregated by cabin and further by seat location.
Amber: assistance request from the lavatory. Steady indicates a passenger in the lavatory has requested assistance from an attendant. Flashing indicates a smoke detector activation.
Green: attendant call, using the cabin interphone to call between interphone stations.

The indication panel is integrated with, and receives event indications from, the Cabin Intercommunication Data System (CIDS). The CIDS is a crucial network on the aircraft responsible for handling crew communications, managing some aspects of the flight attendant displays, operating the speaker system and giving the relevant indications. Calls requiring attention from a flight attendant typically trigger a notification on the attendant loudspeakers and illuminate the relevant light on the ACP (unless already illuminated) in order to draw attention.
Furthermore, for passenger calls, the relevant seat number light in the PSU is illuminated when the attendant button is operated. This allows the request to be localised to the relevant row and side of the aircraft. If an aircraft door is open, the row number flashes -- typically a door open event leads to the PSU row indications to be illuminated to enable embarking passengers to find their seats more easily.

References
Only a small amount of information is publicly available about the ACP, its role in the CIDS and the CIDS more generally. The information here is corroborated by two sources:

A Lufthansa training manual for the A319/320/321 aircraft, published online. See pages 166, et seq.
Jonathan Hurst's site, in particular the excerpts of A320 technical operations manuals, chapter 10 "Cabin".

Any further sources or references, especially for non-A319/A32x models, are gratefully received in the comments.

Not your first answer here, but certainly a good one!

@FreeMan very kind of you to say, thank you!

There's a panel of lights. The blue one indicates that a passenger has pressed a call button. If it were a red light that means the crew is using the interphone to call one another.

Welcome to Aviation.SE! This looks plausible, would you have a documentation/reference to link?


text:
Does the transponder need to be on in the upper limit of Class B airspace?
Is a transponder needed to be turned on to operate above the upper limit of class B airspace?

Yes:
If you combine the two sections provided by SMSvonder Tann and Raaj Tram you get:

14 CFR 91.215 - ATC transponder and altitude reporting equipment and use:

(b)(4) All aircraft in all airspace above the ceiling and within the lateral boundaries of a Class B or Class C airspace area designated for an airport upward to 10,000 feet MSL; and
(b)(5)(I) airspace of the 48 contiguous states and the District of Columbia at and above 10,000 feet MSL, excluding the airspace at and below 2,500 feet above the surface; and

You can't fly over B or C without a transponder.
There are some specific exceptions for aircraft not originally equipped with an electrical system.

Yes.
Generally, Class B airspace extends up to 10,000ft. That said, FAA states that:

All aircraft are required to be equipped with a Mode C transponder
  when flying at or above 10,000 feet msl, over the 48 contiguous states
  or the District of Columbia, excluding that airspace below 2,500 feet
  agl.

Class B airspace does not necessarily extend to 10,000 feet - consider New York, which tops out at 7,000. Transponder operation is still required, just not necessarily by this particular rule.

@NathanG - That's why I started with "Generally". Here's how FAA defines Class B airspace: "Class B airspace is generally airspace from the surface to 10,000 feet MSL surrounding the nation’s busiest airports in terms of airport operations or passenger enplanements.". Source - https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/media/PHAK%20-%20Chapter%2014.pdf.

Fair enough, I was distracted by your FAR citation. 91.215(b)(4) would be a better reference than 91.215(b)(5)(i).


text:
What is the physical explanation for the formation and position of stagnation points on an object moving in a fluid?
I have some questions regarding the stagnation points, both on an airfoil and on a generic object.

What is the physical explanation for the formation of two stagnation points on an object moving in a fluid (e.g. a cylinder)?
Why in potential flow with zero circulation the rear stagnation point is located above the trailing edge of an airfoil (and not below)?

Does the Kutta condition hold even on flatback airfoils like this one? They have a truncated trailing edge, so I think the "infinite velocity" problem would still arise (in theory, of course).
Where is the rear stagnation point located? The image below gives their approximate position, but it's not clear where the rear stagnation point is.

(source)

The forward stagnation point is easy to explain: Imagine the flow path of molecules which approach an airfoil. Some will continue above and some below the airfoil contour. Between them there must be one flow path which will neither go above nor below, but hit the airfoil straight on. The point where it hits the airfoil is the stagnation point.
The rear stagnation point is similarly explained: When the flows from above and below meet, there will be a line dividing both. Continue this line forward until it hits the airfoil: This is where the rear stagnation point is located. A rear stagnation point is only possible in inviscid flow, which exists only in theory. In both points the speed of air relative to the airfoil is zero, and air molecules will never completely arrive at the stagnation point, but slow down on their way into it.
An airfoil of zero circulation and zero camber will have its rear stagnation point right at the trailing edge. Only when you add asymmetry by giving the airfoil positive camber, the flow has to develop an asymmetric pattern which shifts the rear stagnation point away from the trailing edge to satisfy the zero circulation condition. Give the airfoil negative camber, and the rear stagnation point will duly relocate below the trading edge.
The Kutta condition postulates that the rear stagnation point is fixed at the trailing edge. Make the trailing edge blunt and you need to define where precisely your stagnation point will be. Note that your choice of location will influence the circulation at a given angle of attack! The Kutta condition imposes an arbitrarily chosen stagnation point which just helps to explain real flows better. This does not mean that real flows will observe the Kutta condition closely.
If you look for the "real" stagnation point, you should really search for the point where local pressure reaches a maximum, fully knowing that a full deceleration to stagnation will not happen in reality. This can only be outside of the boundary layer, since the energy loss of the boundary layer flow will result in a pressure loss. Now the picture from the second paragraph will help: Find the line separating upper and lower flows and follow it upstream until it hits the airfoil. This procedure will land you straight at the lowest speed section of the boundary layer, and near the airfoil the pressure there is too low. But now move back again and watch how the slow center part of the boundary lower gets accelerated by shear with the neighboring flow. At some point pressure will peak: This is the closest you can come to for a rear stagnation point.
Below I plotted a quick XFOIL calculation of a NACA4418 with a 5% trailing edge gap to get something similar to your airfoil. Note how the $c_p$-value of the viscous flow (solid line) peaks at some point behind the blunt trailing edge. For the outer flow the airfoil looks as if it is slightly longer, the void behind the blunt trailing edge being filled by separated, slow flow. Also note that the upper boundary layer is thicker, shifting the "rear stagnation point" a little up.

XFOIL Plot of a NACA4412 with 5% blunt trailing edge at 4° AoA (own work).

Thank you, excellent answer! One more thing: with "when you add asymmetry", do you also include a symmetrical airfoil with non-zero angle of attack?

@lorenzownd: Yes, absolutely. A symmetrical airfoil at a nonzero angle of attack will need the rear stagnation point shifted away from the trailing edge to stay at zero circulation.